The invention relates generally to the field of analog and digital signal processing circuitry and more particularly to the use of such circuitry in a spark timing and exhaust gas recirculation (EGR) control system for an internal combustion engine.
The desired timing for spark ignition of an internal combustion engine is known to be a complex function of the primary engine variables of engine speed and the vacuum pressure produced by the engine, as well as a function of many other engine variables. Likewise, the amount of exhaust gases which should be reinjected into the internal combustion engine cylinders to insure minimum polution levels and more complete fuel utilization is also a complex function of both of these primary engine variables.
Generally, the ignition spark timing is controlled by a mechanical advance system in which the engine speed, by virtue of centrifugal force, is utilized to alter the mechanical position of a cam which determines the spark timing. In addition, the vacuum pressure produced by the engine is also used to simultaneously mechanically alter the position of this cam and therefore also control spark timing.
Some electronic ignition timing systems have proposed accomplishing the same result by electronic rather than mechanical apparatus. Some of these systems have also tried to control the amount of exhaust gas recirculation in accordance with the engine variables of speed and vacuum pressure. These systems generally implement the complex relationships between the output quantities of desired spark timing and desired exhaust gas recirculation and the input quantities of engine speed and vacuum pressure by either utilizing complex and inaccurate analog signal processing circuitry or complex, costly and redundant digital processing circuitry. The prior art analog processing circuits generally comprise a plurality of zener diodes each breaking down at different analog input levels such that a piecewise linear transfer function is implemented. The prior art digital processing circuits generally comprise separate analog to digital (A/D) converters for each of the primary engine variables and separate digital function generators, preferably read only memories (ROMs), which implement complex function digital output signals for each engine variable. These digital processing circuits then generally add the digital outputs of each of the ROMs in a single accumulator. Thus the digital processing circuits merely provide separately added complex functions of each of the primary engine variables. Therefore, these digital circuits can not implement a complex output function which is the product of complex output functions which depend on each of the primary engine variables.
Most prior art analog to digital converters receive an analog input signal and produce a digital output signal which is directly proportional to the magnitude of the analog input signal. In many applications it is much more desirable to have a digital output which is inversely proportional to the magnitude of the analog input signal. However, prior signal processing circuits have not been able to readily implement this function and have only been able to approximate this function by the use of complex and costly circuitry. In ignition systems, the engine spark timing and exhaust gas recirculation control are functions of engine speed. However, generally only an analog signal proportional to the period of engine revolution is readily produced and the period of engine revolution is inversely related to engine speed. Thus prior art ignition systems must either convert this period related analog signal to its inverse in order to produce the proper spark timing and exhaust gas recirculation control, or the variation of spark timing and exhaust gas recirculation as a function of engine speed must be compromised. Either of these results is undesirable.